101. 对称二叉树

问题描述

给定一个二叉树，检查它是否是镜像对称的。

例如，二叉树 [1,2,2,3,4,4,3] 是对称的。

    1
   / 
  2   2
 /  / 
3  4 4  3

但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:

    1
   / 
  2   2
      
   3    3

解决方案

递归：

class Solution:
    def isSymeetric(self, root):
        if root is None:
            return True
        else:
            return self.isMirror(root.left, root.right)

    def isMirror(self, left, right):
        if left is None and right is None:
            return True
        if left is None or right is None:
            return False
        if left.val == right.val:
            outPair = self.isMirror(left.left, right.right)
            inPair = self.isMirror(left.right, right.left)
            return outPair and inPair
        else:
            return False

迭代：

class Solution:
    def isSymmetric(self, root):
        if root is None:
            return True

        stack = [[root.left, root.right]]

        while len(stack) > 0:
            pair = stack.pop(0)
            left = pair[0]
            right = pair[1]

            if left is None and right is None:
                continue
            if left is None or right is None:
                return False
            if left.val == right.val:
                stack.insert(0, [left.left, right.right])

                stack.insert(0, [left.right, right.left])
            else:
                return False
        return True